Domestic boilers for space heating and domestic hot-water heating are known in a variety of configurations and constructions and generally provide a burner forming a fuel-air mixture which is combusted in a combustion chamber which may form a heat exchanger with a medium to be heated, e.g. water which can be circulated for space heating purposes, a flue for discharging the combustion gases and a variety of controls for monitoring the combustion process.
By and large, such boilers are free-standing and in many cases can be relatively voluminous.
While more modern boilers have tended to occupy less space, generally they are lacking in versatility in the sense that they operate efficiently only at a fixed volume of gas or air flow and are not amenable to modulation to suit different operating conditions.